The analysis of downhole fluid samples is desirable in many oil industry applications. In the prior art this it is typically done by bringing samples to the surface using sealed containers, and sending the samples for laboratory measurements. A number of technical and practical limitations are associated with this approach.
The main concern usually is that the sample(s) taken to the surface may not be representative of the downhole geologic formation due to the fact that only limited sample material from a limited number of downhole locations can be extracted and taken to the surface. Thus, taking samples to the surface is impractical if it is desired to measure the fluid on a dense grid of sample points. Therefore, by necessity the measurements will only provide an incomplete picture of the downhole conditions.
In addition, these samples frequently contain highly flammable hydrocarbon mixtures under pressure. Depressurizing the containers frequently leads to the loss of the gas content. Handling of such test samples can be hazardous and costly. Significant practical problems are further caused by fluid phase changes during retrieval and transport, difficulties in re-creating reservoir conditions and, in general, the significant time delay associated with the laboratory analysis.
It is apparent that the above fluid sampling and analysis process ideally would be preceded or even completely replaced by downhole fluid analysis of as many samples as desired, with the final results instantaneously available at the well site. Nuclear magnetic resonance (NMR) technology is well suited for this purpose, as it enables the user to determine many properties of an in-situ formation fluid without extracting numerous samples. These properties include hydrogen density, self-diffusivity, and relaxation times, T1 and T2. NMR devices, methods and pulse sequences for use in logging tools are described, for example, in U.S. Pat. Nos. 4,350,955; 5,557,201; 4,710,713; 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164 and 6,051,973. The above patents are hereby incorporated by reference.
Direct downhole measurement of certain fluid properties is known in the art. Several commercially available tools can be used to this end. Examples include the RDT™ tool manufactured by Halliburton, the Reservoir Characterization Instrument (RCI™) from Western Atlas, and the Modular Formation Dynamics Tester (MDT™) made by Schlumberger. These tester tools have modular design that allows them to be reconfigured at the well site. Typically, these tools provide pressure-volume measurements, which can be used to differentiate liquids from gases, and are also capable of providing temperature, resistivity and other mechanical or electrical measurements. However, these tools do not generally provide NMR measurements, as discussed above.
The use of NMR measurements to determine downhole formation fluid properties is also known in the field. The first approach to this end is disclosed in U.S. Pat. No. 6,111,408 to one of the inventors of the present invention, which discloses a method and apparatus for making direct downhole NMR measurements of formation fluids. The disclosure of this patent is incorporated by reference for all purposes. The device in the '408 patent, however, generally requires that a portion of the fluid be diverted from the main flow line and be held stationary for the duration of the measurement, which may take about a minute. A possible concern about such use of this device is the occurrence of fluid phase separation due to the diversion from the main flow line and separation due to gravity once the sample has been stagnated. In addition, depending on relative concentrations, the sample chamber may contain only a subset of the phases contained in the flow line.
Accordingly, it is perceived that there is a need for a tester capable of performing direct, continuous-flow downhole NMR measurements that can be used to enhance the quality and reliability of formation evaluation obtained using prior art techniques. Additionally, there is a need to provide a modular NMR downhole analysis apparatus that can be used as an add-on to existing testing equipment so as to minimize the cost of the extra measurements.
In order to more fully appreciate why NMR is important in measurements of this type, the reader is directed to Stokes' equation:D∝kT/η, (k=1.38×10−23 J/K)  (1)
Eq. (1) essentially indicates that the self-diffusion coefficient D is inversely proportional to viscosity η and vice versa. Viscosity and diffusivity are both related to the translational motion of molecules and therefore must be interrelated. At higher temperatures T, a molecule contains more energy and can move faster against a given “friction” η, therefore D∝T. Diffusivity is a property that can be precisely determined by NMR techniques without disturbing or altering the fluid. The relationship D∝T/η has been verified over a wide range of viscosities at different temperatures and pressures by NMR spin-echo experiments. Referring to the prior art references listed below, see, for example, Abragam, 1961.
One has to be more careful with relationships involving the NMR relaxation times T1 and T2. The applicability of expressions of the form,T1, T2∝kT/η,  (2)is more limited than that of Eq. (1). The main reason is that gas/liquid mixtures, such as live oils, relax by more than one relaxation mechanism: dipole-dipole for the liquid phase and mainly spin rotation for the gas phase. In combination, however, NMR relaxometry (measuring T1 and T2) and NMR diffusometry (determining D) are powerful tools to characterize live oils.
With further reference to the prior art list below, the study of NMR relaxation times with respect to oil properties began shortly after NMR was first demonstrated (Bloembergen et al., 1948; Brown, 1961). The practical aspects of how to relate T1, T2 and D to petrophysical fluid properties such as viscosity and gas/oil ratio have attracted interest much more recently (Kleinberg et al., 1996; Lo et al., 1998; Zhang et al., 1998; Appel et al., 2000; Lo et al., 2000). These investigations are significant because NMR relaxometry can be performed at much lower field strengths and with much lower homogeneity requirements than NMR spectroscopy (1,000 ppm v.<1 ppm). The latter seems to be out of reach for downhole applications, but the development of a robust and accurate NMR relaxometry/diffusometry system for downhole use proved to be feasible.
Another favorable factor was the development of the Reservoir Description Tool of Halliburton (RDT; Proett et al., 1999), which is a modular wireline sampling and testing system that can be configured in a variety of tool combinations and can readily accept add-on analysis devices. Advantageously, these tools generally can operate independently of each other. The inclusion of an NMR fluid analyzer in the RDT tool string has the following benefits:
(1) The level of fluid contamination due to, for example, filtrates from water or oil based muds can be assessed continuously by observing the fluid's T1 relaxation time distribution.
(2) Estimates for fluid viscosity and gas/oil ratio (GOR) can be estimated in real time. Viscosity can be derived from either T1 or D, and GOR from a combination of both parameters.
(3) The NMR analysis takes place at true reservoir conditions, removing ambiguities associated with sampling and transport procedures.
(4) The shared measurement principles behind wireline NMR tools and the downhole NMR fluid analyzer encourage the development of integrated formation evaluation methods.
(5) Data from the NMR analysis are processed downhole and the results are viewable in real time. This information can be useful in the selection of further sampling points and for the fine-tuning an open-hole logging program. A simple example of this synergy is the verification of the hydrogen density in the invaded zone by in-situ fluid testing, a number which feeds directly into the porosity reading of both wireline and logging-while-drilling (LWD) tools.
Details of the apparatus and method in accordance with the present invention are provided below. The interested reader is directed for additional background information to the disclosure of the following references, which are incorporated herein by reference for background. For simplicity, in the following disclosure only the first author and date of publication are provided.    1. Abragam, A., 1961, Principles of Nuclear Magnetism, Oxford University Press, Oxford.    2. Appel, M., Freeman, J. J., Perkins, R. B., and van Diik, N. P., 2000, Reservoir Fluid Study by Nuclear Magnetic Resonance, Paper HH: SPWLA, presented at the 41st Annual Logging Symposium, Dallas, Tex., June 4–7.    3. Bloembergen, N., Purcell, E. M., and Pound, R. V., 1948, Phys. Rev. 73, p. 679.    4. Brown, R. J. S., 1961, Proton Relaxation in Crude Oils, Nature 189, no. 4762, p. 387.    5. Coates, G. R., Xiao, L., and Prammer, M. G., 1999, NMR Logging, Principles and Applications, Halliburton Energy Services, Houston.    6. Freedman, R., Sezginer, A., Flaum, M., Matteson, A., Lo, S., and Hirasaki, G. J., 2000, A New NMR Method of Fluid Characterization in Reservoir Rocks: Experimental Confirmation and Simulation Results, Paper SPE-63214: Society of Petroleum Engineers, presented at the b 75th Annual Technical Conference and Exhibition, Dallas, Tex., October 1–4.    7. Kimmich, R., 1997, NMR Tomography, Diffusometry, Relaxometry, Springer-Verlag, Berlin.    8. Kleinberg, R. L., and Vinegar, H. J., 1996, NMR Properties of Reservoir Fluids, The Log Analyst, Nov.–Dec., p. 20.    9. Lo, S.-W., Hirasaki, Kobayashi, R., and House, W. V., 1998, Relaxation Time and Diffusion Measurements of Methane and n-Decane Mixtures, The Log Analyst, Nov.–Dec., p.43.    10. Lo, S.-W., Hirasaki, G., House, W. V., and Kobayashi, R., 2000, Correlations of NMR Relaxation Times with Viscosity, Diffusivity, and Gas/Oil Ratio of Methane/Hydrocarbon Mixtures, Paper SPE-63217: Society of Petroleum Engineers, presented at the 75th Annual Technical Conference and Exhibition, Dallas, Tex., October 1–4.    11. Prammer, M. G., 1994, NMR Pore Size Distributions and Permeability at the Well Site, Paper SPE-28368: Society of Petroleum Engineers, presented at the 69th Annual Technical Conference and Exhibition, New Orleans, La., September 25–28.    12. Prammer, M. G., Mardon, D., Coates, G. R., and Miller, M. N, 1995, Lithology-Independent Gas Detection by Gradient-NMR Logging, Paper SPE-30562: Society of Petroleum Engineers, presented at 70th Annual Technical Conference and Exhibition, Dallas, Tex., October 22–25.    13. Prammer, M. G., Bouton, J., Chandler, R. N., Drack, E. D., and Miller, M. N., 1998, A New Multiband Generation of NMR Logging Tools, Paper SPE-49011: Society of Petroleum Engineers, presented at the 73rd Annual Technical Conference and Exhibition, New Orleans, La., September 27–30.    14. Prammer, M. G., Drack, E., Goodman, G., Masak, P., Menger, S., Morys, M., Zannoni, S., Suddarth, B., and Dudley, J., 2000, The Magnetic Resonance While-Drilling Tool: Theory and Operation, Paper SPE-62981: Society of Petroleum Engineers, presented at the 75th Annual Technical Conference and Exhibition, Dallas, Tex., October 1–4.    15. Prammer, 2000, Method and apparatus for differentiating oil based mud filtrate from connate oil, U.S. Pat. No. 6,107,796, August 22.    16. Proett, M. A., Gilbert, G. N., Chin, W. C., and Monroe, M. L., 1999, New Wireline Formation Testing Tool with Advanced Sampling Technology, Paper SPE-56711, Society of Petroleum Engineers, presented at the 74th Annual Technical Conference and Exhibition, Houston, Tex., October 3–6.    17. Van Dusen, A., Williams, S., Fadnes, F. H., and Irvine-Fortescue, J., 2000, Determination of Hydrocarbon Properties by Optical Analysis During Wireline Fluid Sampling, Paper SPE-63252: Society of Petroleum Engineers, presented at the 75th Annual Technical Conference and Exhibition, Dallas, Tex., October 1–4.    18. Zhang, Q., Lo, S., Huang, C. C., Hirasaki, G. J., Kobayashi, R., and House, W. V., 1998, Some Exceptions to Default NMR Rock and Fluid Properties, Paper FF: SPWLA, presented at the 39th Annual Logging Symposium, Keystone, Colo., May 26–29.